Underreamer Apparatus and Method

ABSTRACT

A wellbore underreamer has a tubular body, a central body cavity, and pivotally mounted blades in slots on the periphery of the tubular body. A translatable piston in the central body cavity is urged away from cam surfaces on the blades by a biasing spring. The piston has an orifice restricted fluid passage and translates in response to fluid pressure determined by the selected piston orifice and biasing spring. Fluid circulates through the piston fluid passage and fluid passages in the underreamer while piston movement is restrained by the spring. At a predetermined fluid pressure, the piston moves to engage the blade cam surfaces to pivotally extend the blades outward from the blade slots. The blades are held in the slots by a spring biased ball until they are extended by the piston.

PRIORITY

This application claims priority to U.S. provisional application Ser. No. 61/837,004 filed by Applicant on Jun. 19, 2013 for Underreamer Apparatus and Method, the entire content of which is incorporated by reference.

FIELD OF THE INVENTION

This invention pertains to downhole equipment for oil and gas wells. More particularly, it pertains to an underreamer apparatus for use on a drillstring, coiled tubing, or mud motor and, more particularly, this invention relates to an apparatus for enlarging a borehole. This apparatus is used to enlarge or clean out a wellbore below a wellbore restriction such as the end of a casing or tubing string.

BACKGROUND OF THE INVENTION

During drilling or workover of a wellbore, it is often deemed necessary to lower an underreamer tool and tool string through a narrow diameter section of tubing or casing then deploy the cutting blades of the underreamer tool to increase the cutting or cleaning diameter of the wellbore. It is also necessary to pump fluid through the tool string to cool mud motors, lubricate the drillstring, and/or maintain well control while the tool string travels to the desired depth. Because pumping fluid through the tube string often results in the premature opening of the blades of many underreamer tools, it would be an advantage to an operator if an underreamer tool would allow for pumping fluid at a known flowrate through the underreamer without the potential for premature blade opening.

During the use an underreamer is subjected to tremendous vibration. This vibration may lead to the weakening or loosening of the pivot pins securing the underreamer blades and substantially increase the likelihood these pivot pins will fail leaving the underreamer blades loose in the wellbore. If the underreamer blades are loose, drilling can no longer continue and the underreamer blades must be fished from the wellbore costing time and money. It would be an advantage to an operator if an underreamer tool would have a secondary means to retain the blade pivot pin(s).

During the use the blades of an underreamer are covered or coated with mud, clay or other debris making these blades less efficient. It would be an advantage to an operator if an underreamer tool was provided with a means to direct fluid onto the cutting surface of each blade to flush away cuttings from the underreamer blades as cutting is advanced to enhance the efficiency of the cutting action.

Similarly, debris is often collected in the blade pockets or recesses of an underreamer. This collection of debris in the blade pockets or recesses can create problems as debris lodged in the blade pocket may prevent the cutting blades from closing or collapsing fully. If the cutting blades are not fully closed within the blade pockets, the extending blades may prevent the underreamer from being pulled back through any previously encountered wellbore restriction, the extending blades may cause the underreamer to become stuck in the wellbore.

A serious problem is created when an underreamer is stuck at a wellbore restriction and often the drillstring must be cut and the underreamer must be left in the wellbore. When an underreamer is left in the wellbore further drilling cannot proceed unless the underreamer can be removed or the well is sidetracked. Both required costly and time consuming procedures. It would be an advantage to an operator if an underreamer tool was provided with a means to direct fluid onto the blade pockets or recesses to flush away cuttings from the underreamer blade pockets or recesses to allow the blades to be fully retracted into the underreamer tool.

Most underreamer tools have no means to retain the cutting blades within the tool when the blades are in a closed or un-open position. Such tools simply rely on gravity to keep the blades in a vertical or closed position. This reliance on gravity works well when drilling vertical wells, but not as well when drilling horizontal wells, as is becoming common. During horizontal drilling the cutting blades of such underreamer tools have a tendency to be unexpectedly deployed and opened which presents unnecessary complications in the drilling process. It would be an advantage to an operator if an underreamer tool was provided with a means, other than gravity, to retain the cutting blades within the tools to minimize occurrences of unexpected cutting blade deployment and opening.

There is also no means in the prior art of cleaning debris from blade pockets or recesses. This can create problems when debris becomes lodged in the blade pocket, preventing the blades from closing or retracting fully. When the blades are not fully retracted, the extending blades may prevent the underreamer apparatus being pulled back through wellbore restrictions cleared when the blades were fully retracted and cause the underreamer to become stuck in the wellbore at the uncleared restriction. A struck underreamer tool can create a very serious problem often remedied only by cutting the drillstring just above the underreamer so that the underreamer is left or dropped in the wellbore. If this occurs the dropped underreamer will likely prevent further drilling and requiring the well to be sidetracked, which is a costly and time consuming procedure.

Consequently, there is a need for an improved underreamer apparatus having the aforesaid advantages that may be used by an operator to enlarge and clean out a wellbore below a wellbore restriction.

SUMMARY OF THE INVENTION

The present invention is for an underreamer apparatus that satisfies the aforementioned needs. The underreamer apparatus is comprised of a tubular housing having a central bore or body cavity and a plurality of elongated slots around the housing periphery. A pivotable blade having an eccentric cam surface at the pivot end is positioned within each elongated slot. The blade is pivotally retained in each elongated slot by a pivot pin and the blades are allowed to pivot about the pivot pin. Each pivotable blade is held in place within the housing slots and is restrained from pivotal movement by means of a spring biased ball and ball detents in the blade. Contact with the cam surface of the blades will pivot the blades on the pivot pin, move the ball from the blade ball detents, and extended the blades radially outward from the elongated slots to place the blades in an open position.

The periphery of these blades creates a cutting structure provided with a hard cutting surface such as a cutting surface comprised of exceedingly hard material such as carbide, polycrystalline diamond composite (“PDC”), hard metal alloys such as tungsten carbide alloys, hard metal materials such as Borium® as manufactured by Stoody Deloro Stellite, Inc., or the like, or a combination thereof.

The underreamer apparatus is provided with a spring biased translatable piston to move against the cam surface of the blades and pivot them to an open position. The piston is positioned within the central bore or body cavity of the housing and moves or translates upward and downward in the housing in response to fluid flow and pressure and the biasing force provided by the spring. The piston may be ported and the spring rates on the biasing springs may be varied to provide for piston movement at a desired fluid flowrate. Translating the piston downward in the housing against the biasing force of the spring will move the piston against the cam surface of the blades and pivotally extend the blades radially outward from the housing slots to an open position for underreaming.

The housing is configured for attachment to a pipestring also having a central bore through which fluid may be introduced. The pipestring may be a drillstring or a coiled tubing string. If required for a particular drilling procedure, the housing may be attached to and rotated by a mud motor attached to the pipestring. The fluid circulated though the pipestring may be a liquid, gas, or a combination thereof.

For use the underreamer apparatus may be attached to a pipestring having a central bore in communication with the body cavity of the apparatus. When so attached, fluid introduced into the central bore of the pipestring will circulate into the body cavity of the underreamer apparatus. When the flowrate of the fluid in the body cavity reaches a predetermined specified flowrate, the piston position in the body cavity of the underreamer apparatus is urged forward (downward in relation to the top of the borehole) into contact with the cutting blades. Once sufficient hydraulic forces are applied, the cutting blades pivot about the pivot pin and begin to open. The force from the opening blades then urges the blade retaining balls from the blade ball detents are into their respective cavities. The blades then continue to open outwardly until the upper surface of the blades contact and engage the tubular housing. At this point, the blades are fully deployed and the apparatus rotated to begin underreaming in the wellbore.

When in use the underreamer apparatus is threadedly attached to the lower end of a mud motor attached to the lower end of a drillstring. Fluid (drilling mud) is circulated from a pump at surface through the drillstring to the mud motor. The fluid circulating through the mud motor will spin or rotate the underreamer apparatus.

The apparatus will be configured so the fluid entering the apparatus housing from the mud motor will move the piston downward at a desired fluid pressure against biasing force of the spring and against the cam surface of the blades to pivot the blades on the retaining pins to extend the blades from the housing to an open position. The apparatus has a plurality of fluid exit ports so fluid may be circulated through the wellbore. The exiting fluid is used to clean and cool the blades and the blade slots while underreaming is taking place. The underreamer apparatus may be threadedly connected directly to the drillstring and rotated from surface in those applications that do not require mud motors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal bottom end view of the underreamer apparatus with blades closed showing section lines F-F.

FIG. 2 is a longitudinal cross-section top view of the underreamer apparatus with blades closed shown in FIG. 1 cut through lines F-F.

FIG. 3 is a longitudinal bottom end view of the underreamer apparatus with blades open showing section lines G-G.

FIG. 4 is a longitudinal cross-section top view of the underreamer apparatus with blades open as shown in FIG. 3 cut through lines G-G.

FIG. 5 is a longitudinal bottom end view of the underreamer apparatus with blades closed showing section lines H-H.

FIG. 6 is a longitudinal cross-section elevation view of the underreamer apparatus with blades closed shown in FIG. 5 cut through lines H-H.

FIG. 7 is a longitudinal end bottom view of the underreamer apparatus with blades open showing section line J-J.

FIG. 8 is a longitudinal cross-section elevation view of the underreamer apparatus with blades open as shown in FIG. 7 cut through lines J-J.

FIG. 9 is a detail view of the ball detent as denoted as detail K in FIG. 6.

FIG. 10 is an elevation view of a second embodiment of the underreamer apparatus pivot pin.

FIG. 11 is an elevation view of the underreamer apparatus prior to welding with the apparatus flow ports shown as hidden lines.

FIG. 12 is an isometric view of the underreamer apparatus prior to welding, the apparatus flow ports shown as hidden lines.

FIG. 13 is an elevation view of the underreamer apparatus showing the internal features as hidden lines with the blades closed and with the blade side positioned at the top and bottom.

FIG. 14 is a blade side elevation view of the underreamer apparatus with the blades closed.

FIG. 15 is an elevation view of the underreamer apparatus showing the internal features as hidden lines with the blades opened and with the blade side positioned at the top and bottom.

FIG. 16 is a blade side elevation view of the underreamer apparatus showing the internal features as hidden lines with the blades opened.

FIG. 17 is a longitudinal cross-section view of a wellbore showing the underreamer apparatus with blades open, as shown in FIG. 15, the underreamer being attached to the lower end of a mud motor and with a drilling bit attached to the underreamer lower end.

DESCRIPTION OF THE EMBODIMENTS

FIGS. 1 through 8 show the underreamer apparatus (5) of the present invention. The underreamer apparatus (5), having an upper end (50) and lower end (55), is comprised of a tubular top sub (10) and a tubular housing or body (15). The body (15) has a central bore or opening creating a body cavity (90).

The body (15) is also provided with opposing elongated blade slots (180) positioned at its periphery. A blade (25) is located within each elongated blade slot (180) and is pivotally attached to the body (15) by means of a pivot pin (40). A blade locking mechanisms (110) is provided to releasably hold the blade (25) in a closed position.

The pivot pin (40) will preferably have a left hand thread and will be threadedly engaged to body (15). Applying a thread locking compound to the threadedly engaged pivot pin, such as a thread locking compound manufactured by Henkel Corporation under the Loctite® brand, will prevent the pivot pin (40) from becoming loose or dislodging from the body (15). A secondary right hand threaded pin locking bolt (105), along with a thread locking compound, may also be used to secure the pivot pin (40). Providing the pivot pin (40) and secondary bolt (105) with opposite thread hands help prevent the pivot pin (40) from becoming loose as the threads securing secondary bolt (105) to pivot pin (40) will become tighter if pivot pin (40) backs out of threaded engagement with the body (15). These measures help to ensure the pivot pin (40) remains secure.

The top sub (10) is threadedly engaged to the body (15) via threaded connection (12). The top sub (10) has a threaded connection (60) having a central bore (62) in communication with the body cavity (90). The threaded connection (60) provides means for attaching the top sub (10) and the apparatus (5) to a mud motor or a tubular pipestring such as a drillstring.

A piston (20) is concentrically located within the body cavity (90) of the body (15) and positioned to move or translate longitudinally in the body cavity (90) between the top sub (10) and blades (25). The piston (20) has a sealing element (70) provided to seal the body cavity (90) between the piston (20) and the body (15). Piston (20) is urged or translated longitudinally upward in the body cavity (90) toward the top sub (10) (upward in relation to the upper end (50) of the apparatus (5) and the top of the borehole) by a spring (30) and may be urged or translated longitudinally downward in the body cavity (90) toward the lower end (55) of the apparatus (5) by fluid pressure in the body cavity (90).

Each blade (25) has an eccentric or cam contact surface (26). The cam contact surface (26) is configured such that the longitudinal motion of the piston (70) against the cam contact surface (26) of each blade (25) will pivot the blades (25) about the pivot pins (40) so that the blades (25) will extend radially outward from the slots (180) in the body (15).

Each blade (25) has a cutting structure (27) having a hard, durable cutting surface that will expedite the reaming process. While the cutting surface of the cutting structure (27) may be a hardened steel or other hard metal surface, it is thought that a cutting structure (27) having a hard cutting surface comprised of carbide, polycrystalline diamond composite (“PDC”), tungsten carbide alloys such as Borium®, or a combination thereof, or the like will be utilized.

The piston (20) has a fluid passage (74) extending longitudinally along its central axis that is in communication with a plurality of angled and radially extending fluid ports (75) and a flow restricting orifice (35). The orifice (35) is positioned at or near the top or upper end of piston (20) to restrict the central fluid passage (74). The piston (20) also has a plurality of axial fluid ports (80) circumferentially spaced about the central axis of the piston (20) at its upper end.

Fluid travelling from central bore (62) in communication with the body cavity (90) through the central passage (74) of the piston (20) is dispersed out of the piston (20) through the ports (75) into body cavity (90) and along and through elongated blade slots (180). The fluid travelling through body cavity (90) and slots (180) washes away debris lodged in the body (90) and slots (180) and reduces or eliminates debris that would prevent the blades (25) from closing or retracting into the slots (180). This is critical because the drillstring (P) will become stuck in the wellbore if the blades (25) are prevented from closing or retracing.

A plurality of fluid passageways (150) and (160) run longitudinally though the body (15) around the body cavity (90). Each fluid passageway (160) is connected to a fluid exit port (165) into the body cavity (90) at its lower end. Each fluid passage way (150) is connected to an angled fluid exit passageway (155) exiting the body cavity (90). Fluid ports (80) are in fluid communication with fluid passageways (150) and (160). The radial fluid ports (80) allow fluid to bypass the central fluid passage (74) and orifice (35) of piston (20) to access fluid passageways (150) and (160), which can be seen more clearly in FIG. 6, FIG. 8, FIGS. 11-13, and FIG. 15 and FIG. 16.

Circulating fluid through the drillstring (P) into the central bore (62) and through underreamer apparatus (5) helps lubricate and cool any mud motors (M) or other downhole tools affixed to the drillstring below the apparatus (5) and helps maintain well control. Because it is desirable for operators to circulate fluid through the drillstring (P) and an underreamer apparatus when traveling through the wellbore, a mechanism to prevent the blades (25) of the underreamer apparatus (5) from opening until the desired wellbore depth for blade deployment is reached is required. The piston orifice (35) and the piston biasing spring (30) provide such a mechanism.

The orifice (35) in the central passage (74) of piston (20) is configured with an opening or fluid passage of a desired size. The orifice (35) restricts and regulates the flow of fluid from the central bore (62) of the top sub (10) to allow a desired predetermined limited rate of fluid to be circulated through the central passage (74) of the piston (20). Providing the spring (30) with a spring constant sufficient to restrain movement of the piston (20) in response to the predetermined flow of fluid through the piston will allow fluid to be circulated through the drillstring and the apparatus (5). The blades (25) can then be maintained in the closed or retracted position shown in FIGS. 1 and 2 while the fluid circulates. One or more magnets (45) positioned in the body (15) at the elongated slots (180) may also be provided as an aid in holding the blades (25) within the elongated slots (180).

Once the operator determines that the drillstring has reached a desired depth or position where underreaming is required, the operator may increase the fluid flowrate through the drillstring well beyond the threshold predetermined fluid flowrate through the orifice (35). This increased pressure will move the piston (20) to engage the eccentric or cam contact surface (26) of the blades (25) causing the blades to pivot on pin (40) to open or deploy the blades (25) as shown in FIGS. 3 and 4. When the blades (25) are fully deployed the drilling and underreaming operations may be commenced. Once weight on bit is applied to the underreamer apparatus (5), and thus blades (25), the flowrate can be decreased without the potential for the blades (25) closing.

A longitudinal end view of the underreamer apparatus (5) with the blades (25) in the open position is illustrated in FIG. 3. It can be seen that the blades (25) are parallel to one another but not in the same plane. This mandates that each blade (25) be located in its respective slot (180) so that the extension of the blades (25) can be maximized to maximize the diameter underreaming and the diameter of the borehole. If the blades (25) shared the same slot (180), they would have to be far narrower, restricting the diameter to which the blades may be opened. Also illustrated in FIG. 3 are ports (165) which can be more clearly seen in FIG. 13. Ports (165) provide fluid to a drill bit (B) which is to be threadedly engaged to the lower threaded connection (65) of the underreamer apparatus (5).

FIG. 4 is a longitudinal cross-section top view of the underreamer apparatus (5) with blades (25) open cut through lines G-G shown in FIG. 3. It can be seen that the piston (20) has moved or travelled downward in the body cavity (90) toward lower end (55) of the apparatus (5) causing the blades (25) to open. The ports (75) in the piston (20) are now aligned with the body cavity (90). Again, the fluid travelling through these ports (75) into cavity (90) will wash away any debris or drill cuttings that may have entered into the elongated blade slots (180). This washing ensures the elongated blade slots (180) are cleared of debris to allowing blades (25) to fully retract or close when drilling ceases.

Another advantage to these angled ports (75) is that the fluid is directed away from the pivot pin (40). Often the fluid being circulated through the drillstring (called drilling mud), contains abrasives such as sand or barite. These abrasives are used to weight up the circulating fluid (i.e. make it more dense). Such abrasives may cause severe wear on the underreamer components especially the pivot pin (40) that weakens the structural integrity of the pin (40) and that may lead to premature pin failure. The failure of a pin (40) may cause the blades (25) to be separated from the underreamer and dropped in the wellbore as debris. If the pins (40) are dropped in the wellbore, they must be removed or fished out of the wellbore in a subsequent well operation called fishing.

Fishing operations are risky and very expensive due to the cost of the labor and equipment required for the operation, the cost of the associated rig time and drilling delay, and the risk of collapse and loss of the wellbore altogether during these operations. Diverting the circulating fluid away from the pins (40) will eliminate or substantially reduce the contact of abrasive fluids on the pins (40). Eliminating or substantially reducing the contact of abrasive fluids on the pins (40) will substantially reduce the risk of pin failure and the potential loss of the pins (40) in the wellbore.

FIG. 5 is another longitudinal bottom end view of the underreamer apparatus (5) with the blades (25) in a closed position showing section lines H-H for the longitudinal cross-sectional view of the underreamer apparatus (5) illustrated in FIG. 6. A portion of the circulation fluid from the pipestring entering central bore (62) of the underreamer apparatus (5) will travel through orifice (35) in passage (74) and the remaining fluid will travel through ports (80) to enter fluid passageways (150) and (160) not seen in FIG. 6. Passageways (150) direct fluid down the body cavity (90) to exit through passageways (155) in a direction toward the lower end of each blade (25) where the blade cutting structures are located. Passageways (150), (155), (160), and (165) are more clearly illustrated in FIGS. 11-16.

Fluid travelling through passageways (155) cools the cutting structures (27) on blades (25) and also flushes away wellbore debris removed by the underreaming action of the rotating blades (25). Fluid flow through passageways (155) enhances efficient drilling operations. Because heat is one of the primary causes of wear, tear and failure of cutting structures, especially those having PDC cutting surfaces, the cooling flow of fluid from passageways (155) will reduce the build of heat and prolong the life of the cutting structures (27). Flushing away debris removed by the action of the blades (25) prevents the cutting structures (27) from continuing to act on the same material already dislodged in the wellbore which will also decrease wear on the blades (25) and speed up the cutting operation. Fluid flow on the blades (25) from passageways (155) also washes debris from the cutting structures (27) to prevent the cutting surface of cutting structures (27) from being coated and clogged with debris which will reduce the cutting performance of the blades (25).

Because all material being removed during the drilling operations must be circulated out of the wellbore and back to surface to provide a well formed wellbore, any loose debris remaining after underreaming operations are performed will increase the risk of that the debris being lodged around the drillstring (P) and creating a “plug” that will cause the drillstring (P) to become stuck in the wellbore. A stuck drill string is both dangerous and expensive. A “plug” will prevent a prudent operator from circulating fluid through the drilling string into the wellbore because doing so would increase pump pressure to dangerous levels and create an unsafe well control issue. The fluid diverted from the apparatus (5) through passageways (155) will wash away loose debris as the wellbore is progressed during cutting and allow the debris to be carried out of the wellbore as the fluid is circulated. This will enhance the operator's ability to maintain control of wellbore pressures and reduce the risk of one of the causes of a well blowout.

FIG. 9 is and enlarged view of the blade locking mechanisms (110) taken from FIG. 6 with the blades (25) in a closed position. The blade locking mechanism (110) is comprised of a ball (125) retained in a ball cavity (126) machined into the periphery of the body (15) perpendicular to each slot (180). A spring (120) is provided to urge into the ball (125) into a retention recess (175) provided in each blade (25) to secure the blade (25) in the elongated slot (180) on the body (15). A set screw (115) is used to secure the ball (125) and spring (126) assembly within the body (15). The ball (125) is prevented from escaping ball cavity (126) via retaining lip (127). The diameter of lip (127) is smaller than that of the ball (125), such that a portion of ball (125) protrudes from cavity (126). When the blades (25) are closed, the ball (125) fits securely into retention recess (175) of the blades (125) to retain blades (25) in the closed position as shown in FIGS. 5 and 6. The spring (120) may be any suitable spring and may be a plurality of stacked spring washers or disc springs, a coiled spring, or any other suitable spring device.

When forces from the piston (20) on the blades (25) are sufficient to pivot the blades (25) on pins (40) so that the blades (25) begin to extend or open, the ball (125) is urged against the spring (120) and is forced into cavity (126). When the ball (125) is positioned within cavity (126) the retaining force of the ball (125) in retaining recess (175) of the blades (25) is reduced to allow the blades (25) to open fully as shown in FIGS. 7 and 8. When forces from the piston (20) on the blades (25) are decreased so that the piston (20) moves upward due to the biasing force of the spring (30), the blades (25) will close into the slots (180) and the ball (125) will then re-engage ball retention recess (175) to secure and hold the blades (25) in position within the blade slots (180) on the periphery of the body (15) as shown in FIGS. 5 and 6.

This ability to re-secure blades in a closed position in the elongated slot (180) after use will enhance the effectiveness of the underreamer apparatus (5). Many underreaming devices typically use O-rings located around the circumference of the body and around the blades to hold the blades closed. When the blades open, the O-ring rolls upward and out of the way. Once the blades close, there is no means to retain the blades in the closed position. Some underreaming devices utilize shear pins or screws to retain the blades in a closed position. When the blades of these devices are opened, the shear screws retaining these blades are sheared and there is no means to re-secure the blades when the blades are returned to a closed or retracted position.

An alternate embodiment of the pivot pin (40) described above is shown as pivot pin (100) in FIG. 10. In this embodiment of the pin (100) a groove (130) is provided on the pin shank. This groove (130) is fitted with a corresponding snap ring or circular clip as a secondary means for retaining the pivot pin in position on the body (15).

FIG. 11 and FIG. 12 illustrate the method of manufacturing the underreamer body (15). The body (15) begins as a blank (135) which is formed by components (140) and (145). Component (140) is a solid piece of steel machined to the shape as seen in FIG. 11. Component (145) is also a solid piece of steel machined to fit component (140) with fluid passageways (150) and (160) drilled as blind holes to the required depth. Component (140) is fitted to component (145) such that a gap (170) is left between each component. Once fitted together, components (140) and (145) are adjoined by welding gap (170) to form a unitary component. The body (15) as shown and described in reference to the previous drawings is then machined to final dimensions.

The fluid passageways (150) and (160) are revealed when the body cavity (90) of the body (15) shown in FIG. 6 is machined. When the blank (145) is machined as described to create the body (15), fluid entering the underreamer apparatus (5) will have access to passageways (150) and (160). This process of manufacture is necessary to provide fluid passageways (150) and (160) without compromising the integrity of threaded connection (12) connecting body (15) to top sub (10).

FIG. 13 is an elevation view of the underreamer apparatus showing the internal features as hidden lines with the blades closed and with the blade side positioned at the top and bottom. Passageways (150) and (160) can be seen as well as exit passageways (155) and (165). Passageways (155) and (165) and hole (185) can also be seen in FIG. 14. Passageways (165) must be drilled from the outer side surface of body (15) toward the central axis of the underreamer apparatus (5). The outermost area of passageways (165), noted as hole (185), must be welded closed so the fluid travelling through passageways (160) and (165) is prevented from exiting from the exterior sides of the underreamer apparatus (5). This configuration can be seen more clearly in FIG. 16.

FIG. 15 is an elevation view of the underreamer apparatus showing the internal features as hidden lines with the blades opened and with the blade side positioned at the top and bottom. The direction of passageways (155) can be seen oriented in a direction toward the lower end of blades (25). The location of ball retention recesses (175) on blades (25) can be seen in this view. FIG. 16 is an elevation view of the underreamer apparatus (5) with the blades (25) in the open position, again with hidden lines visible. The passageways (150), (155), (160), and (165) can be seen in this view.

A cross-sectional view of a wellbore (WB) showing the underreamer apparatus (5) in a drilling application is displayed in FIG. 17. The tubular top sub (10) of underreamer apparatus (5) is threadedly attached to the lower end of a mud motor (M) via threaded connection (60). The upper end of the mud motor (M) is threadedly attached to the lower end of a drillstring (P). A drill bit (B) is threadedly attached to the tubular body (15) at the lower end of the underreamer apparatus (5).

Fluid (F) (drilling mud) is circulated from a pump at surface through the drillstring (P) to the mud motor (M). The mud motor (M) converts hydraulic power from the circulating fluid into rotational power to rotate the underreamer apparatus (5). The top sub (10) provides means for attachment to a mud motor (M) or a drillstring (P).

Fluid travelling through the mud motor (M) activates piston (20) by moving it downwardly to engage the cam surface (26) of the blades (25) causing the blades (25) to pivot on pin (40) to extend the blades (25) to an open position. The fluid flowrate and pressure required to move the piston (20) to deploy the blades (25) may be adjusted by altering the flow area of the orifice (35) and the spring rate or spring constant K of spring (30) resisting downward piston movement.

Fluid exits the underreamer apparatus (5) at the drill bit (B) as well as exit passageways (155) in the longitudinal fluid passages (150). The exiting fluid is used to clean and cool both the drill bit (B) and the blades (25).

In applications not requiring mud motors (M), the underreamer apparatus (5) may be threadedly connected directly to the drillstring (P) and rotated from surface. 

I claim:
 1. An underreamer apparatus comprising: (a) a longitudinally extending tubular body, said tubular body having a top end, a bottom end, a central cavity, and at least one elongated blade retaining slot; (b) a blade pivotally mounted within said elongated blade slot; (c) a piston positioned within said central cavity of said tubular body; (d) a spring creating a biasing force urging said piston away from said blade toward said top end of said tubular body; and (d) whereby movement of said piston against said biasing force of said spring to engage said blade will thereby pivot said blade radially outward from said blade slot and said tubular body.
 2. The underreamer apparatus recited in claim 2, further comprising: (a) a plurality of longitudinally extending fluid passageways in said tubular body; (b) fluid ports in said piston in fluid communication said fluid passageways in said tubular body and said central cavity of said tubular body; (c) a central fluid passage in said piston in fluid communication with said central cavity of said tubular body; and (d) at least one fluid port in communication with at least one of said longitudinally extending fluid passageways in said body whereby fluid may exit said tubular body.
 3. The underreamer apparatus recited in claim 2 wherein at least one of said longitudinally extending fluid passageways in said tubular body directs fluid to said blade and said elongated blade slot.
 4. The underreamer apparatus recited in claim 3 further comprising a flow control orifice positioned in said central fluid passage of said piston.
 5. The underreamer recited in claim 4 wherein: (a) fluid may be circulated through said piston fluid passage and said plurality of fluid passageways while movement of said piston in said central cavity is restrained by said spring; (b) wherein said piston may be translated against said spring in response to fluid pressure determined by a selected said flow control orifice and a selected said spring; and (c) wherein fluid is provided to said central cavity of said tubular body and said piston by an attached pipestring having a central fluid bore in fluid communication with said central cavity of said tubular body.
 6. The underreamer apparatus recited in claim 5 comprising: (a) a spring biased ball and ball detent combination holding said blade in said blade slot; (b) at least one magnet contained within said elongated blade slot; and (c) a cam surface on said pivotally mounted blade for engagement with said piston.
 7. A wellbore underreamer comprising: (a) a tubular body having a central body cavity and a plurality of blade slots; (b) a pivotally mounted blade in each said blade slot, each said blade having a cam surface and a cutting surface; (c) a translatable piston in said central body cavity, said piston having a fluid passage restricted by an orifice; and (d) a biasing spring urging said piston away from said cam surfaces on said blades.
 8. The underreamer recited in claim 7 further comprising a plurality of fluid passageways in said tubular body around said central body cavity.
 9. The underreamer recited in claim 8 wherein fluid circulates through said piston fluid passage and said plurality of fluid passageways while movement of said piston is restrained by said biasing spring.
 10. The underreamer recited in claim 9 wherein said piston translates in response to fluid pressure determined by a selected size of said orifice and a selected said biasing spring.
 11. The underreamer recited in claim 10 wherein said piston moves to engage said cam surface on said blades thereby pivotally extending said blades and said blade cutting surface outward from said blade slots.
 12. The underreamer recited in claim 11 wherein said blades are held in said blade slots by a spring biased ball and ball detent combination.
 13. The underreamer recited in claim 12 wherein at least one fluid passageway of said plurality of fluid passageways directs fluid through one of said blade slots.
 14. The underreamer recited in claim 13 wherein said tubular body of said underreamer is attached to a mud motor mounted on a drillstring.
 15. The underreamer recited in claim 13 wherein said tubular body of said underreamer is attached to a pipestring having a central fluid bore whereby said central fluid bore of said pipe string is in fluid communication with said central body cavity of said tubular body.
 16. A wellbore underreamer comprising: (a) a tubular body having a central body cavity and a plurality of blade slots; (b) a pivotally mounted blade in each said blade slot, each said blade having a cam surface and a cutting surface; (c) a translatable piston in said central body cavity, said piston having a fluid passage restricted by an orifice; (d) a biasing spring urging said piston away from said cam surfaces on said blades; and (e) a plurality of fluid passageways extending through said tubular body around said central body cavity.
 17. The underreamer recited in claim 16 wherein: (a) fluid circulates through said piston fluid passage and said plurality of fluid passageways while movement of said piston is restrained by said biasing spring; (b) said piston translates in response to fluid pressure determined by a selected said orifice and a selected said spring; and (c) wherein said piston moves to engage said cam surface on said blades thereby pivotally extending said blades and said blade cutting surface outward from said blade slots.
 18. The underreamer recited in claim 17 wherein said tubular body of said underreamer is attached to a pipestring having a central fluid bore whereby said central fluid bore of said pipestring is in fluid communication with said central body cavity of said tubular body.
 19. The underreamer recited in claim 18 wherein at least one fluid passageway of said plurality of fluid passageways directs fluid through one of said blade slots.
 20. The underreamer recited in claim 19 wherein said blades are held in said blade slots by a spring biased ball and ball detent combination. 